Cultures of S. typhimurium isolated from man and animals prior to 1948 and cultures of the same serotype isolated from man and fowls after Jan. 1956 were compared for resistance to several antibiotic substances. Significant results were obtained only with tetracyclines. Whereas none of the 200 older cultures examined was resistant to tetracyclines, 9% of cultures recently isolated from fowls and 5% of cultures recently isolated from man were resistant to the substances. There is a possible relationship of these findings to the inclusion of tetracyclines in poultry feeds.